This is a temporary solution to race conditions between user space
programs acting on the same channel.  The code serializes all
operations on a channel, to prevern chan->user_block to disappear
while being used by another process.

We keep the mutex when going to sleep, which means that only one
process at a time can be sleeping on a cahnnel -- other processes
will be sleeping on the semaphore, thus not obeying O_NONBLOCK.

We'll try to do it more cleanly in master, but this al least fixes
possible oopses in the currently installed systems.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>